Download Chapter 4: Degradation of Surface Water Resources

Most of the usable water in South Africa is abstracted
from rivers: there are no large natural lakes and few
aquifers (Basson 1997, Davies & Day 1998). One of
the major threats to rivers of the country, therefore, is
water abstraction.
River regulation through
impoundment and water transfer has become necessary
for the continued supply of water to the growing human
populations within the country, and there are few rivers
in South Africa that are not affected by weirs, dams or
water transfer schemes (Davies & Day 1998). The
consumption of water, however, is only one side of the
coin, the other being the pollution of freshwater
resources through the discharge of wastes and irrigation
return flows. In terms of water quality, the major
threats to surface water resources have been identified
as salinisation and eutrophication (DWA 1986).
Furthermore, erosion and sedimentation lead to
degradation of surface water resources, and results from
inefficient and unsustainable land-use practices which
have occurred unchecked for decades.
The pressures of population growth, increased
agricultural and industrial activities, and urban sprawl
not only lead to resource degradation, but also the loss
of habitat. Urban wetlands, estuaries and rivers are
particularly threatened by physical destruction, due to
canalisation, hardening of the floodplains for
development, and removal of riparian vegetation. The
following section describes the major causes and effects
of the degradation of surface water resources in South
Africa. In addition, the measurement and extent of
degradation are detailed, and lastly, a summary of
government intervention and policies that aim to
minimise or control degradation, is provided. The
surface water resources referred to in this chapter
include rivers, wetlands and estuaries.
Chapter 4: Degradation of Surface
Water Resources
Catherine Snaddon
4.1 Introduction
The understanding that the quality and quantity of water
standing or flowing within the waterbodies of a
catchment are directly affected by the land-use practices
within that catchment, has brought with it the realisation
that water, soil, vegetation and people need to be valued
and managed together, rather than as separate entities.
This philosophy is embodied in the principles of
integrated catchment management, which aims to shift
away from the narrow focus of management of a single
resource, such as water, or soil, or vegetation (DWAF
1996b, Auerbach 1997, DWAF/WRC 1998). It is in
this context that an effort has been made to present an
integrated assessment of the degradation of the surface
water resources of South Africa, as part of the National
Desertification Audit.
South Africa is a semi-arid country, and its surface
water resources are characterised by variability and
unpredictability (Davies et al. 1995). The average
rainfall, of 497 mm yr-1, is similar to that of a country
such as Canada, but the conversion of rainfall to runoff
is an order of magnitude lower in South Africa, i.e.
8.6% compared to 65.7% for Canada (Alexander,
1985). This conversion figure is reportedly one of the
lowest in the world, after Antarctica and the desert
regions, and is largely attributable to the high
evaporation rates experienced across the country. The
coefficient of variation of mean annual runoff (MAR)
varies greatly throughout the country, increasing in the
more arid parts of the country such as the Karoo, where
ephemeral systems predominate (Davies et al. 1993).
The mean coefficient of variation (Cv) for South African
rivers is 89%, which is far higher than other regions of
the world (Table 4.1). For example, approximately
40% of the length of South Africa’s rivers experience
seasonal flows (O’Keeffe et al. 1992).
This extreme variability is reflected in the
percentage contributions of runoff from the various
major catchments of South Africa. The Orange River
system is by far the largest catchment in South Africa,
but contributes only 13.5% of the MAR, while the
catchments of the east coast collectively contribute
more than half of the country’s runoff (Davies et al.
1993) (Figure 4.1). More than 60% of the country’s
runoff originates from only 20% of the land surface.
Thus, although South Africa possesses adequate
mineral and other natural resources, water is scarce.
Table 4.1 Mean coefficients of variation (Cv) in runoff
for rivers in various regions of the world, for
comparison with South Africa (data taken from Davies
et al. 1993).
Country/region
U.S.A.
Canada
Europe
Victoria State (Australia)
Australia + New Zealand
Africa
Asia
South Africa
37
Number of
rivers in
analysis
72
13
37
10
83
Cv (%)
38
20
22
53
50
23
27
89
Figure 4.1 A map of the major catchments of South Africa, and the percentage contribution of each to the mean
annual runoff (from Noble & Hemens, 1978).
and provide information on the effects that these have
on the use and availability of water, water quality and
the natural structure and functioning of the freshwater
ecosystems.
4.2 Causes and Effects of Degradation
4.2.1 Introduction
Degradation of surface water resources occurs as a
result of unnatural alterations in the quantity and quality
of water entering, standing or flowing in a waterbody,
and in the geomorphology of the waterbody itself. Such
alterations occur naturally, but anthropogenic activities
can change the rate and frequency with which these
fluctuations occur. Surface runoff is determined by
rainfall energy, intensity and duration, and by
vegetation cover, soil characteristics and slope (Dallas
& Day 1993, Gale & Day 1993). Activities which
interfere with these variables will affect surface runoff.
In turn, water quality in a waterbody is governed by
geology, geomorphology, climate and vegetation, and is
directly determined by the quality of runoff (Gale &
Day 1993).
It has been shown that mountain catchment areas are
extremely important for the maintenance of streamflow
in a river. Although mountain catchments occupy 8%
of the surface area of South Africa, these areas yield
49% of the runoff (Wilson 1984). Anthropogenic
degradation of mountain catchments will, therefore,
have a significant impact on water supply.
The following sections describe the major causes of
degradation of surface water resources in South Africa,
4.2.2 Degradation of the riparian zone
The riparian zone of a surface waterbody has loosely
been defined as the area adjacent to the watercourse
itself. There is no fixed extent to the riparian zone, as it
will vary according to the position within and shape of
the catchment; for example, the riparian zone of a
headwater reach is narrower than that on the floodplain.
A typical figure given for the extent of the riparian
vegetation is 5% of the catchment area (e.g. Le Maitre
et al. 1993) while, in a recent model calculating water
use by trees, the area occupied by the riparian zone was
estimated as 10% of the total catchment (Scott et al.
1998).
The relationship between surface water
ecosystems and the associated riparian vegetation has
been noted as an intimate and dynamic one. Vegetation
is an important control variable in the geomorphology
of river channels, while in turn, river flow and sediment
loads affect plant growth (Rowntree 1991).
Human activities within the riparian zone have a
direct influence on the waterbody. Preservation of the
riparian zone has been associated with the maintenance
of water quantity, water quality and the structure and
38
functioning of aquatic and semi-aquatic fauna and flora.
For instance, the integrity of the riparian vegetation has
a direct effect on the quality of water entering and
flowing in a watercourse (Dallas & Day 1993, Le
Maitre et al. 1993, Davies & Day 1998). Riparian
vegetation serves as a physical and biological filter for
sediments and nutrients from catchment runoff, and is
important for stabilisation of banks and soils, thereby
reducing erosion. It has been shown that clear-felling of
riparian vegetation increases water temperatures
sufficiently to affect fish, and leads to increased algal
growth and nutrient levels in waterbodies (Gale & Day
1993). The maintenance of a buffer strip of vegetation
alongside a waterbody greatly reduces these impacts
(Hellawell 1986).
increases to 2.4% and 4.8% for catchments with 500
and 250 mm of runoff. If riparian vegetation uses twice
as much water as other vegetation, the percentage
reduction in runoff approximately doubles in each case.
Invasion of the riparian zone by exotic plant species
Aside from the deliberate planting of commercial
forests alongside watercourses, riparian zones are prone
to invasion by various exotic plant species. Riparian
zones are particularly vulnerable to invasion (Rowntree
1991), as:
 they are exposed to natural and human-related
disturbances, such as trampling of banks through
stock watering, burning of riparian vegetation and
bulldozing of river banks;
 there is, generally, a constant availability of water;
 river flow provides a reliable means of dispersal,
and
 river banks act as effective seed reservoirs.
Afforestation
The early belief was that afforestation of bare slopes of
mountains would stabilise the soils and thus decrease
erosion and increase water yields (Bands 1989). In
1908, C. Braine, an engineer in the Transvaal
Department of Irrigation stated that “Afforestation is of
vital importance for maintaining the permanence of
streams.” (quoted from Wicht & Kruger (1973) by
Bands (1989)). In the early part of this century,
members of the Mountain Club of South Africa were
given pine seed to sow in the upper catchments of
rivers. Afforestation was widely encouraged and was
linked with the objectives of water conservation.
Research has since shown that the opposite is true.
The assumption is now made that the reduction in
runoff is directly proportional to the above-ground
biomass of vegetation in the catchment, and that plants
growing in the riparian zone transpire freely due to the
(generally) sufficient supply of water and, therefore, use
more water than those occurring in the remaining
catchment area (Le Maitre et al. 1993). Riparian
vegetation must have a direct influence on streamflow
as water is drawn from the supply that feeds the stream
(i.e. groundwater and surface runoff), and alterations in
the species composition of the riparian vegetation will
affect streamflow. Afforestation and invasion by exotic
tree species such as hakeas, acacias, gums and pine
significantly reduce water yields in comparison to
natural grassland and fynbos vegetation (Bands 1989,
Le Maitre et al. 1993). This is thought to be due to the
higher evaporation rates found in commercial timber
species and other exotic tree species, in comparison
with indigenous species.
The percentage reduction in runoff attributable to
afforestation of the riparian zone, is dependent on the
proportional use of water by riparian vegetation versus
that used by the remaining vegetation biomass in the
catchment and on climate (Le Maitre et al. 1993).
Under the hypothetical conditions of 800 mm of runoff
without vegetation, and water uptake proportional to
plant biomass across the whole catchment (i.e. riparian
vegetation uses the same amount of water as vegetation
across the catchment), pine trees reduce annual runoff
by 1.5%.
The percentage reduction respectively
Invasive plant species colonise different geomorphic
features (i.e. channel bed, channel bars and shelves,
channel bank or floodplain) within the riverine
ecosystem (Rowntree 1991).
Floating aquatic
macrophytes invade the river channel and rooted
aquatics and herbaceous vegetation tend to colonise
channel bars and shelves (see Section 4.2.8; Other
exotic and invasive species), while woody species
prefer the channel bank and floodplain (i.e. the riparian
zone). Most of the alien invasives are found in the latter
two geomorphic features, and the most notable species
are all woody.
Most of the streambank tree and shrub invaders
result in water loss, compared with plant species that
invade the waterbody itself, which lead to obstruction of
flow (Table 4.2, Rowntree 1991, Gale & Day 1993). In
addition to water loss, however, several exotic tree
species increase bank erosion.
The effect of woody species on bank erosion
depends on the growth form, cover density and the
extent and density of the rooting system. In most
instances, the replacement of grasslands with deeperrooting tree species will lead to the stabilisation of
channel banks. However, many of the exotic tree
species found along the rivers in South Africa have
fairly shallow rooting systems, which do not stabilise
the banks and which cannot withstand spates and
floods. Many species have dense canopies which shade
the soil and prevent the development of an understorey
of marginal vegetation such as the Palmiet reed,
Prionium serratum, which would stabilise the banks.
Many gum species dry out the surface layers of soil
through water uptake, to such an extent that other plants
cannot compete for water, resulting in bare, often sandy
soils that are fairly unstable. Acacia mearnsii, A.
longifolia, A, saligna, Lantana camara and Pinus
pinaster are species which are associated with increased
bank erosion in South African rivers (e.g. Macdonald &
39
Richardson 1986, Versfeld & van Wilgen 1986,
Versveld 1993).
Stands of woody invaders accumulate nutrients and
so they deplete the nutrients available in the soil of the
riparian zone, and entering the water (Gale & Day
1993). Invasion by exotic species will also alter the
food base, on which aquatic fauna and flora depend,
especially in the upper, forested reaches of a river. For
example, the input of leaves into headwaters of rivers in
the fynbos biome peaks during spring and early summer
(King et al. 1987a, b, Stewart & Davies 1990). The
leaves falling into the system are also of a certain
quality and palatability. Exotic invaders provide leaves
of different nutritional quality, palatability, quantity and
timing, thereby altering the nutrient cycles and
disrupting the food web dynamics (Gale & Day 1993).
In summary, the extent to which invasion of the
riparian zone by exotic species will affect channel
characteristics depends on the physical characteristics of
the channel – bed and bank sediments, flow regime,
channel slope and morphology – and on various features
of the species themselves – growth form, age, stand and
canopy density (Rowntree 1991).
Table 4.2 Numbers of exotic invasive species which affect aquatic habitats in South Africa, and
the nature of the impacts (from Rowntree 1991).
Habitat
Streambank
Streambank/Aquatic
Aquatic
Life form
trees and shrubs
herbs
herbs
herbs
Obstruction
11
4
3
16
The effects of fire
Impact
Water loss
17
1
2
13
Stagnation
1
12
1993). The effects on runoff were minimal in the case
where the riparian vegetation remained intact (Britton et
al. 1993), and short-lived when the vegetation was burnt
but regenerated fairly rapidly (Lindley et al. 1988).
It has been shown, however, that forested
catchments, or those with a large build-up of biomass,
are more likely to show marked increases in runoff, and
also in erosion, after a burn (Wilson 1984, Scott & van
Wyk 1992, Versfeld 1993). The build-up of vegetation
intercepts rainfall, thereby reducing water yield; runoff
will dramatically increase after a fire (Wilson 1984).
This was recorded in forested kloofs of the Devil’s Peak
mountain in the Western Cape, and in pine plantations
in the Bain’s Kloof Pass, in the same province. In both
cases, severe erosion followed the fires.
The phenomenon of fire-induced soil water
repellency has been recorded as a determinant of firerelated increases in runoff and erosion (Scott & van
Wyk 1992). It is thought that this occurs as a result of
the vaporisation of hydrocarbons in the soil at high
temperatures, which then precipitate out as a waxy layer
around soil particles.
This increases the water
repellency of the soil. In some instances, very hot fires
can reduce soil water repellency in the top 5 cm of soil,
but produce a repellant layer below this depth; thus, the
non-repellant top 5 cm of soil are washed down the
catchment with the first rains (Professor W. Bond,
University of Cape Town, pers. comm.).
Although most of the research on the effects of fire
on forested catchments has been done on pine
plantations, it is surmised that invasion of catchments
by other high biomass exotic plant species will also
increase the amount of flammable material in a
catchment and thus, increase the risk and effects of fire
(Versveld 1993). Some of the exotic invader tree
Early legislation in South Africa aimed at protecting
catchments from fire, as it was believed that fire,
followed by grazing of early successional grasslands,
inevitably leads to increased erosion and sediment loads
in rivers and streams (Bands 1989). The assumption
was that burning removed the natural cover of
vegetation, thereby exposing the soil to trampling by
animals and compaction by the impact of falling rain.
This reduced the water absorbing capacity of the soil
mantle, and increased erosion. Subsequent research,
especially in the Western Cape and KwaZulu-Natal
mountains, has shown that periodic burning of natural
vegetation in upper catchments maintains the species
diversity and vigour of the vegetation, decreases fuel
loads and thus, the risk of catastrophic burns, and is one
of the most effective management tools for the
maintenance of upper catchments as sources of
adequate and pure quality water (Wilson 1984, Bands
1989).
Research done in upper catchments in the Western
Cape have shown that the effects of fire on a riverine
ecosystem depend directly on the extent to which the
riparian vegetation is burnt. It has been noted that the
riparian vegetation is seldom burnt during fires, and its
integrity is an important factor controlling sediment and
nutrient input after a fire (Wilson 1984, Scott & van
Wyk 1992, van Wyk et al. 1992).
Relatively
undisturbed natural catchments in the mountains of the
Western Cape were fairly unaffected by controlled
burns. Increases in nitrates and other nutrients, and in
sediment loads in affected streams were small and
short-lived, persisting only for the first year after burn
(Lindley et al. 1988, Britton 1990, 1991, Britton et al.
40
species, such as the hakea and acacia species, have seed
regeneration cycles that are adapted to fire, and can
rapidly colonise a burnt site (e.g. Wilson 1984).
the human population, and the pressures of economic
growth. Salinisation of aquatic ecosystems is the
process through which the total dissolved solids or salts
in the water is increased (Williams et al. 1984). It
occurs as a result of both natural and anthropogenic
processes. In many cases, natural salt concentrations
are augmented by anthropogenic sources of salts (e.g.
Davies & Day 1998). Salinisation of surface waters in
South Africa is of major importance as it renders water
unsuitable for human use and can have severe
consequences for aquatic fauna and flora.
High
salinities affect the suitability of water for irrigation; for
example, high sodium concentrations damages the clay
particle structure and permeability of soils (DWAF
1991). The problem is of increased significance in
semi-arid environments. The ions that are of particular
importance in South Africa are sodium and calcium,
followed by divalent cations, and carbonates and
sulphates (Williams et al. 1984). There are three major
sources of ions: terrestrial, atmospheric and
groundwater.
In South Africa, the majority of ions dissolved in
natural waters are derived from mineral weathering
(Dallas & Day 1993). Rocks of the Table Mountain
Series are derived from fairly ancient (ca 500-320
million years old) weathered sandstones in which very
small quantities of leachable salts remain. On the other
hand, the Malmesbury Shales were laid down as mud in
ancient seas and thus, still hold large quantities of
leachable ions (e.g. Davies & Day 1998). The other
source of ions is the atmosphere, with an obvious
increase in air-borne salts towards the coastline. Day &
King (1995) investigated the proportions of major ions
in the rivers of South Africa and Lesotho, and
delineated four broad categories of ionic proportions
which are linked to the geological and climatological
features of the landscape. Category 1 describes the
rivers of the high-altitude basalt cap of Lesotho and
KwaZulu-Natal, and the Dolomite and Pretoria Series of
the Northern Province, while Category 2 covers the
lower-altitudes in this same area, including waters
flowing over Karoo and Waterberg sedimentary rocks
and igneous rocks of the Basement Complex and the
Bushveld Igneous Complex. Category 3 is widespread,
and is not linked with geological formations, while
Category 4 describes the rivers of the Western Cape
which flow over rocks of the Table Mountain Series,
and also the western arid regions on Karoo sediments,
and the rivers of coastal KwaZulu-Natal on a range of
substrata. The natural waters within categories 1 and 2
are dominated by salts derived from weathering, while
aquatic ecosystems within category 4 are dominated by
precipitation processes when dilute, and evaporation
processes when concentrated (Day & King 1995).
Rivers that have particularly high natural salt
concentrations are the lower Berg River (Western Cape)
and the Great Fish and Sundays rivers (Eastern Cape).
Human activities increase salinities by either direct
application of salts (e.g. mining, industrial and domestic
effluents) or by accelerating the rate of accumulation of
4.2.3 Pollution
Trace elements and heavy metals
Trace elements occur naturally in small quantities in
surface water resources, as a result of geological
weathering. Most of these elements are highly toxic,
but their toxicity depends on several factors, such as the
pH of the water, presence of other metals and
chemicals, and the flow rate and volume of water
(Dallas & Day 1993, Davies & Day 1998). Aluminium,
for example, is highly toxic, but only becomes available
as a toxic and soluble aquo-aluminium ion when the pH
drops below 5. Heavy metals are found in waterbodies
as a result of human activities, especially mining
(Davies & Day 1998). Metals such as mercury,
beryllium, lead, cadmium, nickel and copper are fairly
problematic in South Africa, and several cases of severe
heavy metal pollution have been reported in the mining
areas, such as in wetlands in Gauteng and Mpumalanga
(Coetzee 1995), and estuaries near Richards Bay in
KwaZulu-Natal (V. Wepener, University of Zululand,
unpublished data). In the majority of cases, heavy
metals reach rivers, wetlands and estuaries from point
sources. All of these metals are known to be toxic to all
organisms, but their effects on ecosystems has not been
investigated in great detail. Ecotoxicological work in
South Africa has lagged behind the development of
water quality standards for waterbodies.
Mining activities are the main heavy metal polluters,
as the procedures used to extract the required minerals
release other heavy metals in soluble form (Dallas &
Day 1993). Other polluters include industries that use
or manufacture metals, such as motor car and
refrigerator manufacturers, and even battery and toy
factories. Urban runoff also contains large quantities of
lead, which is added to petrol (Coetzee 1995).
Trace elements and heavy metals cannot be broken
down further, and are, therefore, persistent pollutants.
Many of the dangerous metals form charged ions in the
water column, which then adsorb onto suspended
particles. On settling out, these particles take the
adsorbed metals into the sediment, where they can
remain indefinitely. Storm events and turbulent flows
lead to pulses of toxic metals from the sediment. These
elements can also be concentrated at higher trophic
levels, becoming increasingly toxic to predators and
carnivores. Unfortunately, the disposal of toxic waste is
very expensive and the easy option of illegal dumping is
attractive to mines and industries.
Salinisation
The major water quality problems experienced in South
Africa have been identified as salinisation and
eutrophication (DWA 1986). These naturally-occurring
processes are exacerbated by the continual growth of
41
salts (e.g. as a result of irrigation, or removal of natural
vegetation).
The discharge of salts into aquatic
ecosystems occurs from either point- or diffuse-sources.
irrigation return flows to rivers and groundwater. As a
result of the increased salinities of groundwater,
combined with the clearing of deeper-rooted natural
vegetation, the water table rises, leading to the seepage
of water into rivers and, where groundwater is highly
salinised, capillaric rise of water carries saline water
into the root zone (Flügel 1993). This problem is often
addressed by increased irrigation, and the salinisation
cycle continues. Inadequate drainage of fields increases
evaporation losses and furthermore, evaporation of
water from storages leads to the concentration of salts in
irrigation water (Williams et al. 1984).
The phenomenon of “dryland salinity” results from
dryland agriculture where the natural vegetation is
cleared, to be replaced by shallower-rooting crops. This
problem has been recorded in many of the semi-arid
countries of the world, including Australia, India,
Argentina, and the semi-arid regions of North America
(Flügel 1991). Deep-ploughing and changes in or
destruction of natural vegetation, affect the infiltration
capacity of the soil, and may also mobilise soluble salts.
Other diffuse sources include saline urban runoff
and runoff from mining activities, especially open-cast
mining. Sulphur dioxide and nitrogen oxides released
from coal- and oil-burning power stations dissolve in
water and form acids which then leach calcium and
magnesium from the soils, further increasing the
salinisation of surface waters (Davies & Day 1998).
River regulation through the construction of dams
and weirs exacerbates the problem of increased
salinities. Variable releases of water from regulating
structures around the country lead to fluctuations in
salinity levels in watercourses. For example, the
Pongolapoort Dam on the Pongolo River in northern
KwaZulu-Natal regulates the flow of the river to such
an extent that flushing flows no longer wash the
accumulated salts from the pans on the floodplain. To
add to this problem, the Makatini Flats Irrigation
Scheme alongside the Pongolo River overlies substrata
that contain “fossilised” seawater (Davies & Day 1998).
Before 1981, water released from the Gariep Dam on
the Orange River, through the Orange-Fish transfer
tunnel into the Great Fish River catchment effectively
reduced salinities in this river (O’Keeffe & de Moor
1988, du Plessis & van Veelen 1991). After 1981,
increased irrigation in the catchment cancelled out this
dilution effect.
Point-source
Saline industrial and mine effluents are commonly
discharged into waterbodies. Heavy industrial activities
in Gauteng are known to discharge effluents with
concentrations of salts that are approximately
1000 mg l-1 higher than that of supply water (Williams
et al. 1984). Similarly, saline effluents from mines can
exceed 10 000 mg l-1, and it has been estimated that this
source of salts accounts for a third of the salt load of the
Gauteng area (previously the PWV-area). For example,
salinisation of the Vaal Barrage as a result of pointsource salinisation led to a total dissolved salt
concentration of 470 mg l-1, rising to 740 mg l-1 for 10%
of the time. Water use problems are experienced at
400 mg l-1, and water with a salt concentration over
1500 mg l-1 is unsuitable for most uses (DWAF 1991).
It has been estimated that 60% of the salts entering the
Vaal Barrage are produced by only four mines (Davies
& Day 1998). Waters impounded by the Vaal Dam
have also experienced an increase in total dissolved
solids of 2.5 mg l-1 each year over the past 25 years.
The cost to the taxpayer of these increases in salinities,
in terms of the consequent loss of irrigable land, and
increased purification costs, was estimated in 1984 to be
R78 000 000, for every 100 mg l-1 rise in TDS in the
Vaal River (Davies & Day 1998). In the early 1980s,
farmers using irrigation water from the Vaalhartz
Irrigation Scheme could expect their profit margin to
decrease by R2 700 000 per year if the salinity of the
water were to increase by 200 mg l-1 to rise above a
threshold concentration of 650 mg l-1 (du Plessis & van
Veelen 1991). Other rivers which receive significant
volumes of mining effluent include the Buffalo, Mkuzi,
Wasbank, Mfolozi and Tugela rivers in KwaZulu-Natal.
The process of sewage purification subjects waters
to evaporative concentration, as a result of the recycling
of treated effluent, and is a problem in many urban
catchments in South Africa, such as the Buffalo River
(Eastern Cape) (Palmer & O’Keeffe 1990) and the
Black River in Cape Town (Clayton 1996). On
average, the total dissolved salts concentration of
municipal effluents is 150 to 500 mg l-1 higher than the
water supply (Williams et al. 1984).
Acidification
Diffuse-source
Various industries release acidic substances into rivers,
wetlands and estuaries. Such activities include the
chemical, beer and tanning industries (Coetzee 1995,
Davies & Day 1998). A more dangerous diffuse source
of acid pollution is acid precipitation. Atmospheric
pollutants are a problematic source of ions, especially in
Gauteng and Mpumalanga.
Coal-burning power
stations and other industries release large quantities of
chemicals into the atmosphere, and some of these reach
surface waters through precipitation (i.e. as acid rain),
or as dry fallout (Table 4.3) (Howard-Williams &
The salinisation of aquatic ecosystems in semi-arid
regions as a result of irrigation has been documented as
a significant threat to water resources and their
management, and the management of irrigation schemes
(Flügel 1993, Davies & Day 1998). Salts dissolved in
the irrigation water are combined with the salts derived
from mineral weathering and fertilization of crops, and
these salts remain after the evaporation and
consumption of the irrigation water. This leads to saline
42
Alexander, 1984, Bosch et al. 1986), leading to
acidification and increased salinities. The low pH water
flows over or through rock strata, dissolving and
transporting high concentrations of metals and salts.
For example, increased salinity levels in the Vaal basin
have been attributed in part to atmospheric pollution as
a result of the location of several power stations in and
adjacent to the catchment (Williams et al. 1984).
waterbodies. Abstraction of water from ecosystems for
farming has also led to substantial streamflow
reductions (Gale & Day 1993). Research done in the
Western Cape on the effects of trout farms on riverine
biota, showed a significant impact on invertebrate
community composition downstream of a trout farm
using portapools, as opposed to earthdams for housing
fish (Brown 1996). It was found that effluent from the
farm led to increased quantities of particulate organic
material, nitrate, ammonium and phosphate in the water
column.
Industrial and agricultural effluents contain toxic
organic pollutants, while biocides (herbicides,
insecticides and fungicides) are applied intentionally to
the land and, consequently, to water resources. As with
heavy metals, biocides accumulate in the sediments and
levels increase through bioaccumulation up the food
chain. Concentrations of these toxins that are lethal to
many organisms are fairly difficult and expensive to
detect.
Biocides are used fairly regularly in South Africa, in
the fight against pests such as locusts and red-billed
queleas (Davies & Day 1998). Many of the compounds
used, such as the chlorinated hydrocarbons (CHC, the
active component of DDT) find their way into rivers
and wetlands through the groundwater, windblown soil,
or direct surface runoff. In some rivers of South Africa,
pesticides have been used to control organisms such as
snails, mosquito larvae and blackfly larvae; all of these
are either vectors of diseases or are pests in their own
right. Wetland soils are fairly high in organic and clay
content, which gives them a high potential for CHC
adsorption (Coetzee 1995). Once in the sediment layer,
many of these compounds are transformed biologically
or chemically into substances that are as or more toxic
than the original compound.
Table 4.3 Atmospheric emissions and rates
of wet deposition for the Mpumalanga
Highveld in 1984 (from Tyson et al. 1988).
Emission (x 106 tonnes)
Particulate material
Sulphur dioxide
Nitrogen oxides
Carbon monoxide
Hydrocarbons
Carbon dioxide
Wet deposition (kg ha-1)
Sulphates
Nitrates
Ammonium
0.4
1.0
0.4
0.3
0.3
123.6
16 - 24
4 - 11.4
3.2 - 4.9
Organic pollution
Enrichment of water resources as a result of organic
pollution from sewage and sewage effluents is probably
the most common type of pollution in South Africa
(Davies & Day 1998) (see also below – Eutrophication).
Organic pollutants can be either toxic or, more
commonly, deleterious due to their volume in organic
waste. Organic waste comprises mostly particulate
matter, with high concentrations of dissolved organic
carbon. Dissolved and particulate organic matter are
generated through biological activity, such as
decomposition of dead organisms, and are important
sources of food for detritivores and decomposers. Most
of these organisms require oxygen, and it is this
depletion of oxygen that is particularly damaging to
aquatic ecosystems and their inhabitants. In addition,
the introduction of large quantities of particulate matter
increases the suspended loads and turbidity, and thus
the penetration of light. The growth of heterotrophic
slimes (also known as “sewage fungus”) is a response
by aquatic bacteria and fungal communities to the
availability of dissolved organic carbon and nutrients in
organic waste (e.g. Ractliffe & Brown 1994) and are,
therefore, an indicator of organic pollution.
The main sources of organic pollution are raw and
partially treated sewage, food processing plants, animal
feedlots and abattoirs (Davies & Day 1998). A further
source of organic pollution is the increasing occurrence
of aquacultural enterprises. Land-based aquacultural
activities are generally situated near or next to
freshwater ecosystems, for purposes of water use.
These enterprises generally lead to elevated total
suspended solids concentrations and nutrient levels, due
to the discharge of faeces, urine and excess food into
Eutrophication
Eutrophication has been defined as the enrichment of
waterbodies with plant nutrients, especially nitrogen
and phosphorus, which leads to excessive growth of
algae and floating or attached macrophytes. It has been
determined that phosphorus is the most common
primary limiting factor determining algal growth in
non-eutrophic waterbodies in South Africa, while
nitrogen is commonly limiting in highly eutrophic
systems (Allanson et al. 1990, du Plessis & van Veelen
1991, Dallas & Day 1993). As in the case of
salinisation, there are natural and anthropogenic sources
of nutrients. Natural sources are relatively predictable
and constant, while anthropogenic sources are variable
in source, quality and quantity, and often lead to
excessive enrichment of waterbodies (Table 4.4) (Dallas
& Day 1993, Davies & Day 1998). Lentic (standing)
waterbodies, such as wetlands, estuaries and reservoirs,
are more vulnerable to nutrient enrichment than lotic
(flowing) systems, due to the fact that there is a greater
retention of substances entering standing systems.
Furthermore, the addition of runoff down the catchment
leads to the dilution of nutrient inputs. However, rivers
43
and streams can experience an accumulation of nutrient
loadings downstream if inputs are situated along the
length of the system, and additional enrichment can lead
to eutrophication effects that will visibly affect the
system (Dallas & Day 1998). Problems can also occur
during periods of low flow, and in systems found in the
drier parts of South Africa. Such problems are therefore
exacerbated by water abstraction.
High-nutrient fertilizers are a major source of
eutrophic pollutants (Coetzee 1995) (Table 4.4).
Nitrogen and phosphorus are essential for the
functioning of such fertilizers, and thus agricultural
runoff brings excess nutrients into rivers, wetlands and
estuaries. Livestock feedlots are also a significant
source of nutrient-rich runoff. Effluent from sewage
plants can be high in nutrients as these are not removed
by primary and secondary treatment (Coetzee 1995).
Sewage treatment plants in South Africa effectively
remove most of the nitrogen from sewage, and it is
released as atmospheric nitrogen.
However, the
removal of phosphorus is still a problem, and sewage
effluents even from effective waste-water treatment
plants have high concentrations of this nutrient. A
problem which is common throughout South Africa is
that of inadequate or non-existent sewage treatment and
disposal mechanisms. Informal settlements and poor
municipalities release practically raw sewage into
rivers, wetlands and the groundwater. Examples can be
drawn from Gauteng, where the Olifantsvlei and
Natalspruit receive sewage effluent from the south of
Johannesburg, and KwaZulu-Natal, where the
Mhlangane Swamps receive eutrophic effluent from
sewage works north of Durban.
Domestic and
industrial detergents are a source of phosphorus, and it
has been estimated that 35 to 50% of the total
phosphorus load in urban sewage is attributable to
detergents (DWA 1986).
Further sources of nutrients include domestic
and industrial solid waste disposal sites and industrial
effluent (Coetzee 1995). Sorghum malt, sugar and dairy
industries have been found to release effluents that
contain phosphorus levels that are well in excess of
recommended quality standards. The degradation of
surface waterbodies as a result of eutrophication leads
to impaired aesthetics of natural waters, increased
purification costs, blockage of water transfer routes (e.g.
irrigation canals and pipelines), taste and odour
problems and potential health risks, and a general loss
of habitat integrity and biodiversity (du Plessis & van
Veelen 1991, Harding 1992, Davies & Day 1998). The
trend towards eutrophication of waterbodies in South
Africa is directly linked to increased human populations
and industrial growth.
Plant growth is dependent on several factors,
including nutrient status, light penetration, temperature,
aeration and species concerned (du Plessis & van
Veelen 1991). Thus, the causes of algal blooms and
excessive macrophyte growth need to be assessed with
caution, and synergistic effects need to be taken into
account. For example, algal growth below the Vaal
Barrage would be expected to peak during the summer
months, but this is not so, as increased light penetration
in the clearer water during winter, results in increased
growth during these months.
Unfortunately, many of the plant species that take
advantage of the eutrophic conditions in a waterbody
are problematic, weedy species. These species are
adapted to establishment under these conditions, and
can reproduce and grow rapidly to optimise production.
Two such examples are the planktonic cyanobacteria,
Microcystis aeruginosa and Anabaena spp., which have
reached nuisance proportions in vleis and
impoundments in the Western Cape (e.g. Harding
1992), and in Hartebeespoort Dam. Under some
conditions, certain species of algae can become toxic,
and pose severe health risks. These fast-growing
species tend to outcompete other aquatic plant species,
Table 4.4 Natural and anthropogenic sources of nutrients to waterbodies in South Africa (from Dallas & Day
1993).
Sources
Climatic
Catchment characteristics
Weathering of rocks and soil
Erosion
Rainfall
Variability of runoff
Surface geology
Land form
Sewage effluent
Industrial discharge
Intensive animal enterprises
Detergents
Agricultural surface runoff
Disturbance of soil mantle
Addition of fertilizers
Urban runoff
Point-source:
Anthropogenic
Diffuse-source:
44
Figure 4.2 The distribution of four genera of invasive aquatic macrophyte.
and form thick mats which then prevent light
penetration for other plants within the water column and
in the benthic zone.
Other problem species that take advantage of
eutrophic conditions are the floating macrophytes,
Eichhornia crassipies (water hyacinth), Myriophyllum
aquaticum (parrot’s feather), Salvinia molesta (Kariba
weed) and Pistia stratiotes (water cabbage) (Figure
4.2). These plants are problematic as they block
watercourses, provide habitat for other pest species such
as mosquito larvae, and increase the loss of water from
the surface of the waterbody through transpiration
(Davies & Day 1998). Most of these species originate
in South America, where they are adapted to living on
the surface of lentic waterbodies and slow-flowing large
rivers, such as the Amazon. South Africa lacks natural
standing waters, but the impoundment of many of our
rivers has provided available habitat for these invasives,
in the absence of any competitors (Davies & Day 1998).
Both algal mats and dense rafts of floating
macrophytes effectively prevent light penetration into
the water column, and to the bed of the waterbody
(Davies & Day 1998). Thus, the growth of other plant
species is inhibited, leading to decreased oxygen levels
within the system. Such low oxygen or even anaerobic
conditions can lead to fish and invertebrate mortalities.
4.2.4 Erosion and sedimentation
Erosion and sedimentation are directly influenced by
the geology, topography, vegetation, soil characteristics
and climate of the catchment (King 1996). Erosion of
land surfaces by wind and rain is a natural process, and
much of South Africa is subject to medium to very high
levels of natural erosion (Booth 1994, Davies & Day
1998). Precipitation has been noted as being the most
important determinant of fluvial sediment loads
(McCormick & Cooper 1992). Sediment yields within
a catchment vary in proportion to rainfall, and in inverse
proportion to the density of surrounding vegetation,
while the type of vegetation is also an important
determinant (Langbein & Schumm 1958). Sediment
yields are directly determined by slope and the
erodibility of the soils of the catchment (McCormick &
Cooper 1992, Rooseboom 1992, Rooseboom et al.
1992).
Anthropogenic perturbations within a catchment,
many of which have already been mentioned in this
chapter, will have an effect on erosion and
sedimentation dynamics of aquatic ecosystems. Any
perturbation within the catchment that leads to bank
destabilisation or which alters the soil characteristics of
the surrounding slopes will increase erosion and
sediment loads (Heeg et al. 1986, McCullum 1994).
This would, therefore, include damage and destruction
45
of the riparian vegetation (see Section 4.2.2), noncontour ploughing, ill-timed burning and overgrazing
which leads to denudation of the vegetation (Davies &
Day 1998). The disturbance of surface slopes for the
creation of roads, bridges, railway tracks and cultivated
lands can all lead to increased erosion and high
sediment loads (Coetzee 1995). The cultivation of
certain crops, and invasion of the riparian zone by
exotic plant species, also affect the rate of erosion
within a catchment. For example, in Australia, a 40%
expansion in sugar cane cultivation resulted in threefold increase in sediment yields (Arakel et al. 1989).
Aridification, meaning a drop in groundwater levels,
leads to a reduction in the capacity of soils to store
water, and thus, to an increase in the susceptibility of
soils to erosion (Booth 1994, King 1996). Aridification
results from overgrazing and trampling of soils which
decrease the infiltration capacity of the soil, and from
the replacement of natural vegetation, such as dryland
grasses, with crops with greater water requirements,
such as wheat.
In addition, alterations in the storage capacity of an
aquatic ecosystem can lead to destabilising flows which
accelerate erosion. For example, the cultivation or
destruction of wetlands and sponges can increase flow
variability and thus, erosion (Nilsen 1969, Heeg et al.
1986). Many of the upper reaches of tributaries of
major rivers in KwaZulu-Natal and of the Orange River
in Lesotho are eroded and turbid, probably as a result of
trampling and grazing of wetlands and sponges in the
Drakensberg range (Davies & Day 1998).
Clearing of vegetation for agriculture can often
result in the release of substantial quantities of nutrients
and silt, by altering water and nutrient runoff patterns
and increasing soil erosion. Overgrazing of grasslands
can lead to excessive erosion and sedimentation (Nilsen
1969). In South Africa, as in many drylands, the finer
suspended solids released through erosion are
electrically charged, and they form a perpetually
suspended flocculate which results in permanently
turbid waters (Davies & Day 1998). This phenomenon
is noticeable in rivers in the former Transkei area,
KwaZulu-Natal and in many reservoirs around the
country.
The regulation of the rivers of South Africa has led
to a greater predictability of flow and to reduced flood
peaks (e.g. Davies & Day 1998). A consequence of this
is the encroachment of agricultural lands to the water’s
edge of many waterbodies, and into the channels of
many rivers through the infilling of secondary channels,
and so on. This practice exacerbates flood damage (i.e.
erosion) during extraordinarily high-flow events, as
banks are unstable (Ractliffe et al. 1996).
Sedimentation (or siltation) is the deposition of
sediment load from the water column onto the beds of
rivers, wetlands and estuaries, leading to them
becoming shallower, and spreading over a wider area,
thereby increasing water losses through evaporation
(McCullum 1994, de Villiers 1988). Over the longterm, sedimentation is a naturally-occurring process
which varies with particle size and the rate of flow.
Accelerated deposition of sediment, however, is a
serious problem, as it can lead to the smothering of
entire ecosystems and it can shorten the life span of
reservoirs and irrigation infrastructure (King 1996).
Wetlands and estuaries, and other standing waterbodies,
are more susceptible to deposition, and the estuarine
environment along the east coast of South Africa has
been greatly reduced through accelerated sedimentation.
The presence of floodplains, where river flow rates
decrease, and where there are wetlands and pans,
increases the likelihood of sediment deposition
(McCormick & Cooper 1992).
An increase in
suspended particles of medium to large sizes results in
siltation and abandonment of channels, and the loss of
wetland habitat (Coetzee 1995).
The impoundment of rivers has an obvious effect on
the amount of fluvial sediment reaching the river
mouth. Sedimentation of reservoirs in South Africa was
the motivation behind a focused research effort to
develop a sediment yield map for the country, which
would assist the siting of new dams (Rooseboom, 1992,
Rooseboom et al. 1992). Within an impoundment the
decrease in flow results in the settling out of sediments.
Most of the impoundments in the Free State, Northern
and Eastern Cape, KwaZulu-Natal, Northern Province,
North-West and Mpumalanga are effective silt traps
(Davies & Day 1998). Below a dam water releases tend
to flush the finer sediments from the river bed, as a
result of the sediment-hungry nature of released water
which has deposited its sediment load within the
reservoir, and the increased turbulence of dam-releases.
As a result, sediment yields will increase in the shortterm below an impoundment and, in the long-term, will
decrease once the finer sediments have been removed (a
phenomenon known as bed armouring; Simons 1979).
Soils
Soil is the medium within which the hydrological
processes of a catchment take place. The water
infiltration capacity at the soil surface determines
subsurface water supply and thus, runoff. The rate of
infiltration is itself determined by variables such as the
organic content of the soil, initial wetness, texture and
structure (Bosch et al. 1986). It has generally been
found that naturally well-vegetated catchments have
infiltration capacities which exceed the intensity of
precipitation and thus, soil surface characteristics are
not determinants of the movement of water into
subsurface soils (Bosch et al. 1986). Consequently,
subsurface runoff determines the flow regime within the
river. On the other hand, in semi-arid catchments (i.e.
most of the land surface of South Africa), urban
catchments and catchments with water-repellant soils,
the infiltration of water through the soil surface
becomes a limiting factor, and river flows are
dominated by surface runoff.
The hydrological properties of soils describe the
ability of soils to absorb, store and redistribute water
which is received within the catchment as precipitation
46
or as the lateral movement of soil moisture (interflow)
(Schulze 1985). Schulze (1985) and Schulze et al.
(1985) have described over 500 soil series within South
Africa according to their hydrological properties, and
provided a preliminary grouping of soil series in terms
of runoff response and soil water retention.
Any anthropogenic activity which leads to the
degradation of soil characteristics will affect associated
aquatic ecosystems by altering runoff and water
retention. Soil degradation has been recorded as a result
of afforestation, burning of catchment vegetation,
inefficient agricultural practices and overgrazing. More
detail on soil degradation is provided elsewhere in this
report.
occurs in perceived abundance to where it is required
for use (Figure 4.3a, b).
4.2.5 River regulation
Dams represent a disruption of the river continuum,
in that they create a lentic environment within a
continuous lotic system. The flow regime is inevitably
altered and water quality conditions downstream of an
impoundment are changed from the natural conditions
(Table 4.5) (e.g. Ward & Stanford 1983, Ward et al.
1984). Water held behind a dam or weir is generally
clear due to the settling out of suspended particles.
Thus, water released from both surface- and deeprelease structures is erosive or “sediment-hungry”.
Furthermore, the water released is nutrient-poor as a
result of the uptake of nutrients by plants and other
organisms within the impoundment.
The topography of South Africa and the uneven
distribution, in space and time, of rainfall across the
country has led to an uneven distribution of available
water. Most of the metropolitan and industrial growth,
which is associated with mineral deposits and harbours,
has occurred some distance from water sources. In
addition, some of the country’s irrigated lands are
situated in areas where water once was abundant but is
now scarce. This leads to the current situation where
water is stored in impoundments in order to ensure a
continued supply throughout the year, and a
proliferation of water transfer schemes which transport
water, in tunnels, pipelines and canals, from where it
Planktonic groups, such as small crustaceans, fare
well in impoundments, and are found in high
concentrations below dam outlets around South Africa.
Plankton is a ready food source for riverine organisms,
and its occurrence below dams leads to a predominance
of filter-feeding organisms in downstream reaches
(Snaddon 1998).
Other downstream effects of
impoundment
include
alterations
in
oxygen
concentrations, which are increased if water is released
from a surface structure, and reduced when water is
released from a greater depth (Table 4.5). Temperatures
and discharges are variable below dams depending on
the release structure and the season.
Figure 4.3a A map of the registered dams in South Africa.
47
Figure 4.3b The major inter-basin water transfer schemes of South Africa. 1, Tugela-Vaal Scheme; 2,
Orange River Project; 3, Komati Scheme; 4; Usutu Scheme; 5, Usutu-Vaal Scheme; 6, Slang River
Scheme; 7, Lesotho Highlands Water Project; 8, Caledon-Modder Scheme; 9, Amatole Scheme; 10,
Riviersonderend-Berg-Eerste River Government Water Scheme; 11, Palmiet River Scheme; 12, MooiMgeni Scheme; 13, Mzimkulu-Mkomaas-Illovo Scheme; 14; Tugela-Mhlatuze Transfer Scheme; 15,
Mhlatuze Scheme; 16, Eastern National Water Carrier; 17, North-South Water Carrier; 18, Grootdraai
Emergency Scheme; 19, Sabie River Government Water Scheme.
Table 4.5 Some downstream effects of impoundment, comparing surface and deep releases (adapted from Ward et
al. 1984).
Variable
turbidity
Surface-released water
reduced
Deep-released water
reduced
silt and sediment load
reduced
reduced
erosive capacity
increased
increased
nutrients
reduced
reduced less
plankton
increased
increased less
dissolved oxygen
saturated
anoxic to supersaturated
temperature
increased
decreased
discharge
variable
variable
The ecological effects of inter-basin water transfers
(IBTs) are little known, but current work indicates that
they have a significant impact on the rivers involved
(Snaddon & Davies 1997, 1998). Donor systems
experience a reduction in runoff and altered flows,
while recipient systems gain often unseasonal
discharges which also disrupt natural flow patterns.
Associated with these changes in water quantity and
flow regime is a host of water quality implications,
especially where water is impounded before transfer,
and where catchments of vastly different character are
48
linked through transfer tunnels. For example, in the
Western Cape, water is impounded in Theewaterskloof
Reservoir, before it is transferred, via pipelines to the
neighbouring Berg River catchment and, eventually, the
city of Cape Town. This seasonal transfer leads to an
830-4500% increase in discharge in the recipient Berg
River during naturally low-flow months, while the pH
and conductivity of receiving reaches of the river are
significantly higher during this period (Snaddon 1998,
Snaddon & Davies 1998). This degradation in water
quality is of concern, as it has implications for the
riverine biota and downstream users.
The regulation of rivers is of particular importance
in a semi-arid country (Thoms & Walker 1992, Walker
& Thoms 1993, Davies et al. 1993). In semi-arid
environments, flows are extremely variable in terms of
timing, duration and magnitude, and aquatic systems
and their biota are adapted to this variability (see
Section 4.1 - Introduction). River regulation, through
impoundment and water transfer, removes this
variability, with severe implications for the system
(Davies et al. 1993). For example, the reduction in
frequency and magnitude of high flow events in a river,
as a result of impoundment, leads to a reduction in
sediment transport, and the ability of a river to prevent
the encroachment of reeds and other rooted
macrophytes into the river channel (e.g. Ractliffe et al.
1996, Palmer 1996).
The discharge of effluents and stormwater into
aquatic systems also leads to changes in flow volumes
and frequencies, and is considered here as a form of
river regulation. Many seasonal aquatic systems,
including rivers, wetlands and estuaries, have been
converted to perennial systems through the continuous
discharge of effluents. This is a common occurrence in
urban areas, where surface waters lie adjacent to water
treatment works and where hardened surfaces increase
runoff into receiving waterbodies. In many instances, it
has become necessary to canalise sections of river and
even estuaries to prevent flooding in urban areas due to
the increased discharges from treatment works and
stormwater runoff. In the case of estuaries, this
regulation alters the nature of the system, such as the
drainage patterns and saltwater intrusion (e.g.
Heinecken et al. 1982).
agricultural use, and through pollution of water
resources as a result of discharge of wastes and erosion
of sediments. Generally, the availability of water and of
suitable croplands should determine the distribution of
human populations; however, in semi-arid countries this
is not the case, as water is often a limiting factor.
Furthermore, South Africa’s previous political regime
exacerbated the impacts of population pressure by
concentrating a vast majority of the population in parts
of the country where available natural resources could
not sustain the subsequent levels of consumption and
pollution (e.g. van Riet et al. 1997, Davies & Day
1998). In South Africa, areas of high population
densities encompass both metropolitan and rural parts.
Water use in South Africa is dominated by
irrigation, which uses 54% of water consumed. This is
followed by environmental water requirements – 19% urban and domestic uses – 11% - and mining and
industrial use – 8% (Basson 1997). Most of the
irrigation occurs in the dry catchments of the country,
such as the Orange River basin, the Crocodile/Limpopo
basin, lower Vaal basin, the Sundays/Great Fish basin,
and the Western Cape.
Afforestation water
requirements are included in the irrigation total, but
occurs in the wetter parts of South Africa, such as the
Mpumalanga escarpment and KwaZulu-Natal (see
Section 4.4.2; Table 4.10). Mining and industrial uses
are focussed in the industrial heartland – Gauteng and
surrounds – and parts of KwaZulu-Natal. The usage
sector that is expected to increase its use most rapidly
over the coming years is the urban and domestic
category (Basson 1997). This increase is expected to
reflect population growth, urbanisation, and increased
standards of living and water supply and sanitation
infrastructure.
Abstraction of water for subsistence agriculture and
small-scale irrigation schemes is largely uncontrolled
and runoff and effluent not monitored (O’Keeffe 1986,
de Lange 1994). This sector can also be expected to
grow in the near future.
The effects of abstraction are similar to those
described in Section 4.2.5 – River regulation. The most
important effects of abstraction are the consequent
disruption of the natural flow regime, and decreased
dilution of pollutants.
4.2.6 Habitat loss
The physical destruction of aquatic habitat occurs
through a variety of means. In rivers, canalisation leads
to a loss of riverine habitat, and the hardening of the
substratum (Davies & Day 1998). Added to this
riverine destruction is the bulldozing of banks and the
removal of riparian vegetation. Urban development on
floodplains has meant the loss of wetlands, pans and
estuaries, while agricultural lands have encroached into
aquatic ecosystems. The list is endless, covering
anthropogenic activities in urban and rural parts of
South Africa.
Seasonal ecosystems are particularly at risk, as they
are perceived to be wasted land for most of the year.
Wetlands have also received the brunt of the
destruction, as they occupy fertile land that is ideal for
cultivation and because wetlands generally occur on
floodplains that are flat and suitable for development.
4.2.8 Biotic change
The physical and chemical effects of surface water
degradation described above are reflected in a variety of
ways in the biota of these systems. Anthropogenic
alterations in environmental conditions generally lead to
a loss of species sensitive to perturbation, while species
that can tolerate such conditions, or those that can
colonise gaps left by sensitive species, are found to
proliferate. The overall result is a loss of species
diversity, and a consequent change in biotic community
structure and functioning.
Furthermore, degraded
conditions are often suitable for invasion by nonindigenous species. Anthropogenic activities can also
have a deleterious effect on invertebrate drift in rivers
4.2.7 Population pressure and water use
High population densities have a significant impact on
surface water resources. This is largely through
consumption of water, for municipal, industrial and
49
or on migration and dispersal between aquatic systems,
but this remains largely unexplored in South Africa
(Davies et al. 1993).
In most cases, the responses of aquatic taxa to
anthropogenic disturbances are poorly understood and,
in all cases, responses are varied (Dallas & Day 1993,
Dallas et al. 1994). Information on the effects of
changes in water quality (as a result of pollution, river
regulation and so on – see previous sections in this
chapter) on individuals is detailed in a comprehensive
report by Dallas et al. (1994), and the reader is referred
to this document. The effects of changes in physical
variables such as flow, channel shape and temperature
are less researched, with the exception of work done by
Bruton (1985) on the effects of suspended particles on
estuarine and lentic species of fish.
which investigated the numbers of riverine invertebrate,
fish and riparian vegetation species across South Africa,
the greatest numbers of species were found on the
eastern side of the country, in the Eastern seaboard,
Sub-tropical east coast and Temperate/sub-tropical
north-east bioregions (Table 4.6). The highest levels of
endemism, in terms of the same groups of biota, occur
in the Capensis biogeographic region in the Western
Cape, where there are 40 species of Trichoptera
(caddisfly) that are endemic (Eekhout et al. 1997). This
region is followed by the Sub-tropical east coast region,
which has an endemism of 10%, dominated by fish
species (Table 4.6). Based on these figures, it can be
concluded that degradation of rivers in regions of high
species richness or endemism poses a threat to
biodiversity. This information should be interpreted
with caution, however, as sampling effort varies greatly
throughout the country. For example, there are no data
available for the arid interior.
Loss of biodiversity
There is little consensus on trends of diversity of biota
in the freshwater systems of South Africa. In a study
Table 4.6 Species richness, degree of endemism and listing of species endemic to each biogeographic region in
South Africa (from Eekhout et al. 1997).
Region
Total Number of
species
Number of endemic
species (% endemism)
Arid north-west
97
3 (3)
Arid interior
?
?
Namaqua capensis
125
18 (14)
Capensis
262
59 (23)
Karroid capensis
116
2 (2)
Eastern seaboard
309
10 (3)
Sub-tropical east coast
436
44 (10)
Interior uplands
254
15 (6)
Basutu highlands
163
4 (3)
Temperate/sub-tropical
north-east
336
21 (6)
50
Species endemic to the region
Trichoptera: 2
Fish: 1
?
Trichoptera: 7
Simuliidae (Diptera): 2
Molluscs: 1
Fish: 8
Ephemeroptera: 6
Trichoptera: 40
Simuliidae: 4
Molluscs: 4
Fish: 3
Vegetation: 2
Trichoptera: 2
Trichoptera: 4
Molluscs: 1
Fish: 3
Vegetation: 2
Ephemeroptera: 8
Trichoptera: 13
Simuliidae: 2
Molluscs: 7
Fish: 14
Vegetation: 1
Trichoptera: 13
Molluscs: 2
Ephemeroptera: 1
Trichoptera: 2
Fish: 1
Ephemeroptera: 5
Trichoptera: 7
Fish: 6
Vegetation: 3
Headwaters of rivers are reservoirs of species, where
endemism is relatively high. Anthropogenic activities
in upper catchments all threaten the diversity of these
valuable environments. Many of these activities are on
the increase, such as hiking, camping, whitewaterrafting, and the cut-flower industry (Wilson, 1984). As
yet, however, the headwaters of many South African
rivers remain relatively undisturbed.
Similar information on the species diversity and
endemism of wetlands and estuaries has not been
synthesised, and thus, the threat of resource degradation
cannot be assessed at such a broad, bioregional level.
threats to aquatic ecosystems in South Africa. The
extent to which these variables are altered from the
natural state provides a measure of degradation. For
example, aquatic ecosystems with total phosphorus and
nitrogen concentrations that exceed 1 mg l-1, are
considered to be eutrophic (Coetzee 1995). In terms of
heavy metals, the South African general effluent
standards provide guidelines for acceptable levels of
heavy metal pollution in aquatic systems. Acceptable
levels of cadmium should not exceed 0.05 mg l-1, while
mercury should not exceed 0.02 mg l-1 (Dallas & Day
1993).
In the past, there has been some variation in the
interpretation of chemical data.
For example,
perceptions of salinity and what this meant for the
ecosystem varied according to whether the investigator
was an ecologist or a manager (Table 4.8).
A recent revision of water quality standards for
South Africa has provided guidelines for water quality
standards according to water use (DWAF 1996a).
Guidelines are given for the following water use
sectors:
 domestic
 recreational
 industrial
 agricultural: irrigation, livestock watering and
aquaculture
 aquatic ecosystems
Other exotic and invasive species
Invasive predatory fish such as the largemouth and
smallmouth bass, Micropterus salmoides and M.
dolomieu, and the brown and rainbow trout, Salmo
trutta and Oncorhynchus mykiss, have had a significant
impact on indigenous fish species of the Western Cape
and KwaZulu-Natal (Skelton 1993). Trout and bass
were introduced in the late 1800s, into streams where
indigenous fish species were few. Indigenous species
lack natural defences against these exotic predators, and
the introduced species have few competitors and
predators. Another problematic introduced species is
the carp, Cyprinus carpio, which stirs up bottom
sediments while feeding, thus destroying benthic
habitats and increasing turbidity. This species is now
widespread throughout South Africa, with the exception
of mountainous areas and parts of the lowveld.
A freshwater snail, Physa acuta, is another exotic
species which has been recorded in substantial numbers
in a variety of aquatic ecosystems in South Africa. This
species has very successfully invaded streams and
wetlands in the Western Cape. The species is tolerant
of polluted waters (Davies & Day 1998).
The water quality guidelines for livestock watering,
for example, records a target water quality range of 0 0.01 mg l-1, and a range of 0 – 1 mg l-1 for mercury.
The guidelines for aquatic ecosystems, however, give
an acceptable range of cadmium concentrations of 0.15
– 0.40 mg l-1, for soft to very hard waters. Acceptable
levels of mercury for aquatic ecosystems should be less
than 0.04 mg l-1. The new water quality guidelines
provide useful indicators of degradation of surface
water resources, according to the user.
A commonly-used index for the rapid assessment of
water quality of rivers in South Africa, the South
African Scoring System, Version 4 (SASS4) (Uys et al.
1996), makes use of the assumption that the state of the
river’s invertebrate inhabitants reflects the overall state
of the water quality of the system. Briefly, this method
makes use of a scoring system that has been developed
by freshwater specialists. The system allocates a score
to each aquatic invertebrate taxon (at the level of
family), according to its occurrence and sensitivity to
water quality perturbations (such as the presence of
pollutants) - a high score is given to a rare taxon that
occurs in pristine waters (Chutter 1994, 1995, Dallas
1995). Invertebrates from all habitats at a given point
on a river are collected and identified, and a cumulative
total score is calculated for that site. In addition, the
total number of taxa and the average score per taxon
(ASPT) is calculated, and these values give an
indication of the types of taxa occurring at the site. In
4.3 Measurement of Surface Water
Degradation
The types of degradation described in Section 4.2 fall
into two broad categories: degradation of water quality
(e.g. pollution) and physical degradation (e.g. river
regulation). Both of these categories lead to the
degradation of various attributes of the aquatic biota.
There is some overlap between the physical and water
quality chemical categories; for example, afforestation
and fire affect both water chemistry and runoff. The
following sections deal with the measurement of water
quality, physical degradation and biotic change.
4.3.1 Water quality
The quality of water within surface water resources is
an important indicator of degradation, and this is
attained through the measurement of various chemical
variables. Table 4.7 provides a summary of chemical
variables commonly used in the measurement of the
51
this way, a number of sites along the length of a river,
or between rivers can be compared. This index is
sensitive primarily to organic pollution (Chutter 1994).
A number of water resource agencies in South
Africa are making use of this method, to provide an
indication of the condition of rivers within their
jurisdiction
or
catchments
(Umgeni
Water,
KwaZulu/Natal; Rand Water and Institute for Water
Quality Studies (DWAF), Gauteng and Mpumalanga;
CSIR, Gauteng; H. Dallas, Freshwater Research Unit,
UCT, pers. comm.). Guidelines for rating water quality
condition, based on SASS4 total scores and ASPT
scores have been provided by Chutter (1995) (Table
4.9).
water quantity (e.g. due to water abstraction) and
channel shape (e.g. due to sedimentation). In this
context, recent research has shown that aquatic
ecosystems are adapted not only to a certain volume of
water, but also to a particular flow regime. The
frequency and duration of discharges in a system
maintain the functioning of that system, and thus the
biotic assemblages that inhabit it.
In the past,
compensation flows from dams (if allocated at all) were
set at a constant minimum volume throughout the year,
which often did not meet the seasonal requirements of
the downstream river reaches. Degradation of surface
water resources in South Africa frequently occurs as a
result of the disruption of the natural flow regime and
the abstraction of water. It has become increasingly
important to be able to quantify the flow requirements
of rivers, so that these can be met.
4.3.2 Physical degradation
Physical degradation refers to unnatural changes in
variables such as temperature (e.g. due to pollution),
suspended solids (e.g. due to erosion), flow regime,
Table 4.7 Chemical variables that are currently monitored in South Africa.
Rivers
dissolved oxygen
oxygen saturation
pH
conductivity
total nitrogen
ammonia
nitrate+nitrite
total phosphorus
dissolved reactive phosphorus
permanganate
silicon
total alkalinity
faecal coliforms
salinity
turbidity
sulphate
dissolved organic carbon
range of heavy metals
total dissolved solids
Inland vleis
dissolved oxygen
oxygen saturation
pH
conductivity
total nitrogen
ammonia
nitrate+nitrite
total phosphorus
dissolved reactive phosphorus
Estuaries
dissolved oxygen (surface and bottom)
biological oxygen demand, chemical
oxygen
demand
pH
conductivity
silicon
total alkalinity
faecal coliforms
salinity
turbidity
sulphate
dissolved organic carbon
range of heavy metals
total dissolved solids
chlorophyll a
silicon
total alkalinity
ammonia
nitrate+nitrite
total phosphorus
salinity
turbidity
sulphate
dissolved organic carbon
range of heavy metals
total dissolved solids
sodium
potassium
chloride
Table 4.8 Perceptions of salinity, with values expressed as mg l-1 (from Williams et al. 1984).
Water managers
Freshwater
ecologists
Fresh
300
Slightly saline
300 – 700
Saline
700 – 1500
Highly saline
1500
3000
3000 – 10 000
10 000 – 100 000
100 000
52
Table 4.9 Guidelines for the assessment of biological condition of rivers in South Africa, from Chutter (1995).
SASS4: South African Scoring System version 4; ASPT: average score per taxon; biotope: physical habitat type,
with its associated biotic component.
SASS4 Total Score
ASPT
Water Condition
For all rivers in South Africa, except Western Cape rivers where the pH<6:
>100
>6
Water quality natural, biotope diversity high.
<100
>6
>100
<6
50-100
<6
ASPT
variable
Water quality natural, biotope diversity reduced.
Borderline case between water quality natural and some
deterioration in water quality, interpretation should be based on the
extent to which SASS4 exceeds 100 and ASPT is less than 6.
Some deterioration in water quality.
<50
Major deterioration in water quality.
For those Western Cape rivers in which pH<6:
>125
>7
Water quality natural, biotope diversity high.
<125
>7
>125
<7
60-125
<7
Water quality natural, biotope diversity reduced.
Borderline case between water quality natural and some
deterioration in water quality, interpretation should be based on the
extent to which SASS4 exceeds 125 and ASPT is less than 7.
Some deterioration in water quality.
<60
<7
Major deterioration in water quality.
There is a growing body of research on the
measurement of the flow requirements of rivers in
South Africa (e.g. King & Tharme 1994, King et al.
1995).
However, there has been no overall
measurement of the extent to which these requirements
have been degraded throughout South Africa.
In terms of channel shape, research by fluvial
geomorphologists has led to the conclusion that river
channel morphology is determined by two important
sets of variables; these are the catchment variables and
site variables (Rowntree 1991). Catchment variables
include all those features which influence runoff and
sediment loads within the river, while site variables
determine river channel processes and shape. Important
site variables include the slope of the catchment, bed
and bank characteristics and riparian vegetation. The
geomorphology of a waterbody is important for the
biota that inhabit the system. Different morphological
units provide habitat for different species assemblages
(e.g. Rowntree 1991). Thus, the maintenance of the
shape of the channel, with its diversity of habitats, is
essential for the maintenance of aquatic ecosystems.
The extent of degradation of the morphology of a
system is measured by habitat indices. Two such
indices are currently used for rivers in South Africa
(Uys et al. 1996, Dallas 1997):
Habitat Assessment Matrix (HAM)
A further method used for rivers, HAM, relates to the
information on the type of substratum, habitat diversity,
velocity and depth, effects of scouring and deposition,
bank erosion, presence of vegetation, etc.
The development of methods for the assessment of
the quality of habitat in wetlands and estuaries lags
behind that for rivers. There appear to be few indices
available for these systems, apart from some records of
the total loss of wetland habitat.
4.3.3 Biotic change
It has been realised that an assessment of the extent to
which various biotic attributes have changed from the
natural condition, provides valuable information on the
extent of degradation, as the biota reflect the state of
that system in ecological terms. At present, common
measures of biotic degradation in surface waters include
those of:
 species richness, measured as numbers of species,
and which gives an indication of the loss or gain of
species;
 various diversity indices, which incorporate
measures of equitability (i.e. how the total number
of individuals is divided between taxa) and species
richness;
 abundances, usually measured either as individuals
per unit area, or as biomass; and
Habitat Quality Evaluation (HABS1)
HABS1 scores are based on the number of habitats,
such as riffles, marginal vegetation, gravel, sand, etc.
53

There are no similar methods for the evaluation of
the conservation status or state of wetlands and
estuaries.
presence versus absence data.
Species lists are invaluable sources of information
on the biota of ecosystems, and these should not be
entirely replaced by indices that combine information.
For example, a high biomass or abundance of
individuals where species richness is low is an
immediate indicator of resource degradation.
There are other community measures of the overall
condition of the biota of a river system. The SASS
index described above (Section 4.3.1) assesses the state
of riverine invertebrates and the extent to which these
are affected by changes in water quality, while two
further indices, described below, assess the fish biota
and the state of the riparian vegetation (Uys et al. 1996):
4.4 Extent of Degradation
4.4 Introduction
The availability of information on the extent of surface
water degradation in South Africa is extremely variable.
Some of the main causes of degradation are welldocumented and their extent and prevention are a
priority, while others continue to spread across the
country without our knowledge or quantification.
Information for the former homelands and for
communal areas is scarce and thus, there is a bias in the
following sections towards the urban and commercial
farming areas. The lack of information for rural and
communal areas is of concern, and needs to be
addressed at a national level.
Furthermore, few
attempts have been made to provide an overview of
surface water resource degradation across the nation,
where all causes of degradation are synthesised. Thus,
there are few maps of degradation for the country. The
following sections focus on the extent of each type of
degradation described in Section 4.2, and a final section
provides an overview of surface water degradation
according to current information.
A further bias that requires mention, is that towards
rivers and stream (lotic) rather than wetlands and
estuaries (lentic). It is probable that this bias is due to
the fact that rivers are our main source of water and
thus, there has been a research emphasis on their
properties and protection.
Index of Biotic Integrity (IBI)
The IBI provides a measure of the state of fish
communities in freshwater ecosystems. It is based on a
variety of variables affecting fish, which relate to the
species richness and composition, trophic composition
(different feeding groups), and the abundance and
overall condition of the fish.
Riparian Vegetation Index (RVI)
The RVI provides a qualitative assessment of the
conservation status of riparian vegetation.
The
maintenance of the riparian corridor is important for the
functioning of the riverine ecosystem, and removal of
important components of the riparian vegetation will
have a direct effect on water quality and flow. The RVI
requires the collection of information on the physical
dimensions of the potential versus actual extent of the
vegetated riparian zone, the species composition,
richness and size/age structure of the riparian
community, and other parameters such as the extent of
erosion, invasion by exotic species, grazing, browsing
and harvesting.
4.4.2 Degradation of the riparian zone
Afforestation
The minimum rainfall required for viable plantations of
most tree species used in afforestation enterprises has
been established as 800 mm yr-1. This, accompanied by
the fact that good agricultural lands are considered
unsuitable for afforestation, means that the lands
targeted for afforestation are the high rainfall,
mountainous marginal agricultural areas extending from
Northern Province, down through Mpumalanga,
Gauteng, KwaZulu-Natal, Eastern and Western Cape
(Table 4.10). This belt corresponds with the mountain
chains which are South Africa’s principal water source
areas.
A recent estimate of the total cover of
commercial timber plantations (pine, wattle, eucalypt
and other hardwoods) in South Africa gives an area of
1.5 million ha, which represents 1.2% of the land
surface (Scott et al. 1998). Mpumalanga Province has
the highest proportion of land – 7.2% - under timber
plantations.
Thus, on an area basis, forestry is
considered a minor land-use, but due to the fact that
plantations occur in high rainfall areas which are
important water sources, commercial forestry needs to
be considered as a significant water user. Commercial
4.3.4 Overview of surface water degradation
A conservation importance map produced by O’Keeffe
(1986) classified rivers according to biotic and physical
state, and the extent to which rivers have been modified
from their natural state. This map serves to highlight
areas of high conservation importance.
A method for the determination of the conservation
status of rivers is in use in South Africa (Kleynhans et
al. 1988). Conservation status refers to the number and
severity of anthropogenic perturbations to a river, and
the damage they inflict on the system.
These
perturbations can be abiotic (such as roads, housing,
weirs) or biotic (e.g. invasive plants and animals). The
method involves a visual assessment of sites along the
length of a river, and scores are given to each site.
However, the results generated from the use of this
method across the country have not been integrated into
an overview of riverine degradation.
54
Table 4.10 Area of commercial plantations in primary catchments of South Africa, and the reduction in mean
annual runoff attributed to plantations (data taken from Scott et al. 1998).
Runoff reduction (%)
Total flow
Low flow
Catchment area
(ha)
Plantation area
(ha)
Limpopo
10 873 287
20 994
0.9
1.8
Olifants
7 350 308
88 055
3.7
11.2
Vaal
19 628 409
1 666
0.01
0.01
Orange
37 824 180
957
0.02
0.01
Olifants (W Cape)
4 906 252
1 000
0. 2
0.2
Namaqualand
2 850 622
0
0.0
0.0
W Cape coast
2 524 374
16 442
1.7
3.2
Breede & SW Cape coast
1 551 872
5 031
0.4
1.2
Gouritz
4 153 434
184
0.03
0.01
716 825
75 616
9.4
12.3
3 473 106
9 027
1.4
1.7
River system
S Cape coast
Gamtoos
Port Elizabeth region
Sundays
E Cape coast
Great Fish
Border coast
261 157
6 055
2.7
2.8
2 122 532
0
0.0
0.0
530 807
628
0.1
0.3
3 022 811
7 640
0.5
1.9
791 831
14 891
1.4
2.9
Great Kei
1 048 308
23 385
1.6
2.8
Transkei region
4 648 239
203 045
2.8
3.4
S KwaZulu-Natal
1 829 157
199 994
4.8
5.8
Tugela
N KwaZulu-Natal & Mpumalanga
highveld
Mpumalanga escarpment
2 902 399
22 628
0.4
0.5
4 507 461
403 150
5.9
8.8
2 857 158
336 294
14.8
22.4
121 734 527
1 436 684
3.2
7.8
South Africa
plantations are estimated to reduce mean annual
streamflow by 3.2% and low (dry season) flows by
7.8%.
The reduction in flow attributable to
afforestation varies throughout South Africa, with the
highest reductions recorded in Mpumalanga. In this
province, almost 10% of total annual flows and 18% of
low flows are reduced. In catchments where flow is
strongly seasonal, and where total annual runoff is low,
the percentage reduction in runoff attributable to
afforestation is exaggerated (Le Maitre et al. 1993). In
Northern Province, for example, small areas of forestry
are confined to humid upper catchments, which are the
primary source of low flows in otherwise dry
catchments (Scott et al. 1998). Table 4.10 provides a
breakdown of information on flow reduction associated
with commercial plantations, and illustrates the
variability of this phenomenon.
(The Working for Water Programme, 1998). Other
areas of significant invasion (10 – 20% invasion)
include catchments in Eastern Cape, KwaZulu-Natal,
Mpumalanga and Northern Province (Macdonald &
Jarman, 1984; The Working for Water Programme,
1998). Most of the research on this aspect has been
concentrated in these areas. The total area of invasion
by exotic tree species in South Africa (beyond invasion
of the riparian zone alone) is equivalent to the province
of KwaZulu-Natal, and this area is doubling every 15
years (Dr G. Preston, Working for Water Programme,
pers. comm.).
The mountainous areas of the fynbos region in the
Western Cape have been invaded predominantly by the
Acacia species. Lower-lying areas of KwaZulu-Natal,
and other parts of South Africa have been invaded by
Sesbania punicea and Acacia species.
Invasion of the riparian zone by exotic plant species
The greatest extent of invasion of the riparian zone by
exotic species (> 20% invasion) has occurred in the
Western Cape, between Hermanus and Cape Infanta
55
diffuse-source pollution, as point-source pollution
effects are negligible during flood events.
4.4.3 Pollution
Heavy metals and trace elements
Surface water resources that are near heavy mining and
industrial areas of the country are threatened by heavy
metal and other trace element pollution.
The
occurrence of severe heavy metal pollution has been
recorded near the mines of Gauteng and Mpumalanga.
In 1996 in Gauteng, for example, the only Ramsar site
in the province, the Blesbokspruit wetland, turned red
due to the discharge of mine effluent into the system.
Ferrous oxide levels increased 250 times above that
permissible by South African water quality standards
(Nel 1997).
Salinisation
A fairly recent estimate of global salinisation was that
approximately a third of all irrigated areas is affected by
this problem. In South Africa, salinisation of surface
waters is considered to be a hazard or potential threat in
all regions east of the Drakensberg (Williams et al.
1984) (Figure 4.4). Salinity distribution has been
mapped by du Plessis & van Veelen (1991), and this
indicates a number of areas in the country with salinities
that are considered extremely high by water managers
(see Section 4.3.1), and no longer fresh by ecologists
(i.e. > 3000 mg l-1). Surface waters of the arid corridor
on the south-eastern coast and arid Karoo have
particularly high salinities (Day & King 1995). The
map also highlights a number of areas in the country
where there are insufficient data; these coincide with
parts of the northern Cape, the Karoo, the Agulhas Plain
and the former Transkei.
There is definite evidence that salinities are on the
increase in many of the river basins of South Africa.
This is certainly true for the Vaal River (Williams et al.
1984, du Plessis & van Veelen 1991), and this has been
linked to the increased socio-economic development in
the catchment. There seems to be little evidence of a
reversal of this trend.
Rivers most affected by
anthropogenic salinisation are situated in Gauteng and
Mpumalanga, where industrial and mining effluents and
coal-burning power stations predominate.
Rivers that are experiencing substantial increases in
salt loads as a result of irrigation practices are the Berg
and Breede rivers in the Western Cape, and the Sundays
and Fish rivers in the Eastern Cape. In the Sundays
River TDS values exceed 1000 mg l-1 for most of the
time. In the neighbouring Great Fish River catchment,
a two-fold increase in irrigated lands, from ca 13000 ha
in 1981 to ca 28000 ha in 1988, resulted in an increase
in total dissolved salts from less than 1200 mg l-1 to ca
1800 mg l-1 over the same period (du Plessis & van
Veelen 1991).
Salt concentrations in the outflow of the Vaal Dam
have increased over the last 40 years from a
concentration of less than 100 mg l-1, to over 150 mg l-1,
with increased dissolved salts concentrations peaking
during flood events. This indicates that this is due to
Figure 4.4 A map of salinities (measured as total
dissolved solids, TDS) across South Africa (from du
Plessis & van Veelen 1991).
Acidification
The effects of acid precipitation have been recorded in
rivers in Gauteng and Mpumalanga (Davies & Day
1998). Wetlands that have been affected by acid rain
pollution include those surrounding the gold mines in
Gauteng, those receiving runoff from the coalmines to
the east of Johannesburg, and those in the Tugela and
Mfolozi catchments in KwaZulu-Natal (Coetzee 1995).
Organic pollution
The problem of organic pollution is concentrated in and
around urban areas. Several of the country’s rivers are
global examples of severe pollution (Davies & Day
1998). The Jukskei/Crocodile river system flows
through the city of Johannesburg, collecting treated
sewage from several parts of the city. Phosphorus loads
in this system have reached 418 tonnes yr-1, over the
past few years. Hartebeespoort Dam, on the Crocodile
River, has suffered several phytoplankton blooms as a
result of organic pollution, leading to “hyperscums”
(collections of dead and living cells) which can be
several metres deep. The Buffalo River in the Eastern
Cape receives treated and untreated sewage from three
large urban areas along its length – King William’s
Town, Zwelitsha and Mdantsane). This has led to tenfold increases in phosphorus and nitrogen levels.
Eutrophication
Information on the extent of eutrophication of surface
water resources is limited to standing waterbodies. An
assessment of the trophic status of 35 reservoirs in
South Africa indicated that 6% of the reservoirs had a
chlorophyll-a concentration that exceeded 30 µg l-1 (i.e.
severe eutrophication) for 50% of the time (DWAF
1991).
In addition, 11% of the reservoirs had
concentrations between 10 and 30 µg l-1, which
56
indicates incipient eutrophication problems. Urban and
industrial development was considered to be the most
important factor affecting eutrophication, while
agricultural activities and erosion also contributed to the
trophic state of the waterbodies.
Examples of
impoundments that are in a eutrophic state as a result of
urban runoff and effluent are the Hartebeespoort and
Roodeplaat dams in Gauteng (Davies & Day 1998).
The extent of eutrophication can also be assessed by
looking at the spread of invasive floating macrophytes
which fare particularly well in eutrophic conditions.
Four species are of major concern in South Africa, these
are
Eichhornia
crassipies
(water
hyacinth),
Myriophyllum aquaticum (parrot’s feather), Salvinia
molesta (Kariba weed) and Pistia stratiotes (water
cabbage). These species are widespread throughout
South Africa, particularly the water hyacinth and
Parrot’s feather (Figure 4.2).
unpublished data). The total volume impounded by
these smaller structure remains unknown.
An estimated 9% of the MAR is transferred between
catchments (Petitjean & Davies 1988). Both transfer
schemes and dams are clustered around the industrial
areas in Gauteng and KwaZulu-Natal, and the
metropolitan centres of the country (Figure 4.3). Figure
4.5 provides an indication of the extent to which each
province’s economy is dependent on water transfer
schemes. Gauteng is 100% dependent on IBTs, ranging
down to only 20% for Northern Province (van Niekerk
et al. 1996, Basson 1997).
The extent of surface water degradation as a result
of the discharge of effluents and stormwater has not
been measured, but poses a serious threat to systems
that lie within or adjacent to urban centres.
100
90
4.4.4 Erosion and sedimentation
80
70
Gross
Geographic
Product (%)
60
The average annual sediment yields for river
catchments in South Africa range from 10 to more than
1000 tonnes km-2 (DWAF 1991), with estimated totals
ranging from 120 to 233 million tonnes of silt entering
South African rivers each year (McCullum 1994). It is
estimated that in some parts of the country, sediment
yields have increased ten-fold as a result of human
activities. Turbidity of rivers draining many of the
former homelands (especially Venda, Ciskei and
Transkei) and Lesotho indicates that overgrazing and
trampling of land in communal areas has led to
increased erosion.
Rooseboom et al. (1992) produced a revised
sediment yield map for South Africa, which was based
on records of sediment loads of reservoirs and rivers
around the country, and which was extrapolated to the
rest of the country by using data on the erodibility of
soils, slope (length and steepness), rainfall and land-use.
The map (see Figure 6.6 in Chapter 6: Soil Degradation)
indicates that much of the interior and east coast fall
within the moderately high to high sediment yield
classes. In these areas, erosion and sedimentation are
problematic, and surface water resources are threatened.
Figure 4.5 A graphic presentation of the percentage of
South Africa’s Gross Geographic Product (GGP), as an
indicator of productivity per province, that is at least
partly reliant on inter-basin water transfer schemes
(from van Niekerk et al. (1996)).
4.4.5 River regulation
4.4.7 Population pressure and water use
There are very few major rivers in South Africa that are
unregulated by dams, weirs or transfer schemes. Those
that are not impounded, such as the Mzimvubu in
Transkei and the Sabie River in Mpumalanga, are
viewed as potential water sources (van Riet et al. 1998).
A total storage capacity of 27 000 x 106 m3 is
provided by the major dams of South Africa, which
represents more than half of the mean annual runoff
(MAR) (Basson 1997). However, South Africa’s land
surface is scattered with smaller dams, including farm
dams which, due to their small size, were not registered
in the past with the Department of Water Affairs and
Forestry (J. Adams, University of Cape Town,
Areas of high population pressure in South Africa (i.e.
population density exceeding 50 people per km2) fall
mainly along the east coast, encompassing most of the
former Transkei, the area around Pietermaritzburg and
Durban and the Tugela River basin, and most of
Gauteng (van Riet et al. 1997). Other patches of high
population density include the Cape Metropolitan Area
and surrounds and the Saldanha area in the Western
Cape, several former homeland areas in North-West,
Northern Province and Mpumalanga, and the
Bloemfontein and Kimberley metropolitan areas. With
the exception of the Mzimvubu catchment in the former
Transkei, and several patches adjacent to Lesotho and
Swaziland, all of the areas of high population density
50
40
30
20
Northern
Province
Northern Cape
Free State
KwaZulu/Natal
Mpumalanga
Western Cape
Eastern Cape
North West
0
Gauteng
10
4.4.6 Habitat loss
Few estimations have been made of the total loss of
surface water habitat in South Africa. An exception is
work done on wetlands, which has concluded that an
estimated 50% of South Africa’s wetlands have been
destroyed (D. Lindley, Rennies Wetlands Project, pers.
comm.).
57

fall within areas where surface water production (i.e.
runoff) is classified as low or average. Thus, the
surface water resources are under severe stress in these
areas.


4.4.8 Biotic change

The extent of degradation of the biotic assemblages of
aquatic ecosystems in terms of overall loss of diversity,
has not been assessed at a national scale. The
development of indices which measure the state of
aquatic biota (Section 4.3) is expected to provide
information on the extent to which the biota have been
affected by anthropogenic change. However, the
gathering of species data is essential and should be
encouraged nationwide.
the lower reaches of several rivers south and north
of Durban (KwaZulu/Natal);
the lower reaches of the Tugela River
(KwaZulu/Natal);
the lower reaches of the Mhlatuze River near
Richards Bay (KwaZulu/Natal), and
the upper reaches of tributaries of the Crocodile,
Olifants and Vaal rivers surrounding the cities of
Johannesburg and Pretoria (Gauteng).
The river sections listed above all lie within
catchments which are highly populated or which
incorporate areas of intense industrial and mining
activities. Wetlands and estuaries in these catchments
are also those that are most threatened. It can be
concluded that population pressures, mining and
industrial activity are the major causes of surface water
degradation in South Africa.
More recently, a national biomonitoring programme
has been undertaken by the Department of Water
Affairs and Forestry, which aims at assessing the state
of riverine ecosystems throughout the country, using the
indices described in Sections 4.3. This will provide a
more integrated view of riverine degradation.
Furthermore, the rivers of the Western Cape are being
assessed by Cape Nature Conservation, in an attempt to
gain information on the extent to which these rivers
have been degraded from their natural state.
The overall state of wetlands and estuaries still
remains to be assessed.
4.4.9 Overview of surface water degradation
The conservation importance map produced by
O’Keeffe (1986) defined rivers according to five
categories as follows:
Class I: channel and catchment has not been
significantly modified.
Class II: slight but significant changes can be detected,
such as mild pollution, water regulation, the presence of
exotic fauna and flora, increased siltation, disturbed
vegetation, etc.
Class III: substantial changes are apparent, such as
locally severe pollution, dominant exotic species, major
water regulation etc.
Class IV: all natural aspects of the channel and
catchment are badly degraded.
A fifth category (the largest one) was for rivers for
which there was inadequate and insufficient knowledge
to place then in any of the above categories. There is a
lack of information for the rivers of Swaziland and the
former Transkei, and for many of the ephemeral rivers
of the Northern Cape Province.
4.5 Government Intervention
4.5.1 Introduction
South Africa is a water scarce country, prone to
droughts. But, in contradiction to this reality, water is
wasted at almost every stage of its management. Water
harvesting techniques are poor, delivery systems are
wasteful, pricing strategies are not cost-effective or
environmentally sustainable, consumption is high in
affluent areas, pollution control is generally ineffective,
and management is fragmented (King 1996). More
recently, however, the concept of integrated catchment
management has found its way into the planning and
management of surface water resources of South Africa
(e.g. DWAF 1996b;. Auerbach 1997). This concept has
focused water resources management on the integrated
nature of catchments and the fact that perturbations
occurring upstream, or within the riparian zone or
floodplain, will have an effect on the remaining aquatic
system. In addition, it has been realised that the logical
management unit is the catchment, which requires
integrated management in order to address problems
within the catchment. Lastly, the realisation that the
water resources of South Africa will not meet future
demand unless human populations curb their
consumption, is one that will shape water use in the
next century.
Class I category applies mostly to the headwater
reaches of a number of South Africa’s rivers, while
most of the length of the rivers (the foothill zones and
lower river) are considered to fall into Classes II and III.
A few river reaches are classified as Class IV, i.e. badly
degraded. These include:
 a section of the middle reaches of the Berg River
(Western Cape);
 the urban reaches of the Black River flowing
through Cape Town;
 the foothill section of the Breede River before its
confluence with the Riviersonderend (Western
Cape);
 the entire length of the Klein River near Hermanus
(Western Cape);
 the lower Swartkops near Port Elizabeth (Eastern
Cape);
 the middle reaches of the Buffalo River
downstream of the King William’s Town/Bisho
area (Eastern Cape);
58
Below are some measures that have been taken in
the past and more recently by the South African
Government in an attempt to slow down the degradation
of surface water resources.
vegetation of a catchment (e.g. Wilson 1984; van
Wilgen et al. 1992).
4.5.3 Pollution
The discharge of all pollutants – heavy metals, trace
elements, organic pollutants and other nutrients and
salts – is controlled and monitored by the Department of
Water Affairs and Forestry. Water quality standards
were introduced through the Water Act of 1956. There
were two standards – general and special effluent
standards.
General standards applied to most
waterbodies, while special standards were applicable for
waterbodies which were considered to be of higher
conservation status due to the presence of exotic trout in
the systems (du Plessis & van Veelen 1991, Dallas &
Day 1993). This was an end-of-pipe approach which
did not address the resource itself and the effects of
pollutants on the ecosystem, and took no account of the
presence of indigenous communities of biota. In
addition to effluent standards, chlorinated hydrocarbonbases pesticides were withdrawn from use in South
Africa in 1981, as part of the Fertilisers, Farm Feeds,
Agricultural Remedies and Stock Remedies Act (1947).
However, these compounds are persistent, and residues
can have an effect up to 22 years after application
(Coetzee 1995).
As discussed in Section 4.3.1, the water quality
standards of the past have been superceded by the water
quality guidelines, which were developed in 1996, as
part of the National Water Law review process (DWAF
1996a) and the development of the Water Services Act
(1997). Guidelines were developed for all categories of
user. The new system is more refined and provides
greater detail on all pollutants. The addition of water
quality guidelines for the aquatic environment itself is a
positive step towards the protection of surface water
resources. However, the water quality guidelines are
still at an interim phase, and the effluent standards
mentioned above are still in effect. The Water Act
(1998), which was effective from 1 October 1998,
places the responsibility for and costs of the removal of
pollutants and the restoration of affected ecosystems at
the door of the pollutant (Government Gazette 1998).
The development and refinement of modelling
techniques has improved the monitoring of water
quality perturbations in surface water resources and of
the effectiveness of management options (e.g. du Plessis
& van Veelen 1991).
4.5.2 Degradation of the riparian zone
Afforestation
The Mountain Catchment Areas Act (1970) was
proclaimed in order to protect the country’s main
sources of water (Wilson 1984). This Act affected both
private- and state-owned land, and the authority to
enforce it rested with the then Department of Forestry.
The Act restricted the planting of exotic timber species
in upper catchments, and gave complete protection to a
number of mountain catchments. This was one of the
first steps towards the protection of land and resources
under a single authority.
Under the Water Act of 1998, plantations are
classified as water users, as they lead to streamflow
reduction. As such, commercial timber enterprises
require permits for water abstraction.
Invasion of the riparian zone by exotic plant species
The primary purpose of the current Working for Water
Programme, which operates under the auspices of the
Department of Water Affairs and Forestry, is the
removal of exotic vegetation from catchments, in order
to increase runoff (Commission on Sustainable
Development, 1998). This Programme was initiated
due to overwhelming evidence that exotic tree species
use a much higher proportion of water within the
catchment than natural riparian vegetation.
This
initiative aims to remove exotics on a continual basis,
and monitor the effects of the removal on rivers and
other aquatic environments. There are no data, as yet,
on the effectiveness of the scheme.
The Programme was funded a total of R33 million in
1995/96 and at least R50 million in 1996/97. By the
end of 1996, 33 000 ha were cleared of exotic
vegetation throughout South Africa.
The effects of fire
The major determinant of the effects of fire on surface
water resources is the extent to which the riparian
vegetation is burnt. In the past, however, there has been
no special protection afforded to the riparian zone, and
agricultural lands and urban development have
encroached to the edges of waterbodies. In many cases,
no buffer strip of vegetation or even open ground exists.
The new Water Act (1998) includes the riparian
zone in the definition of the natural resource, which is
given protection through the ecological Reserve, and
through resource quality objectives which will be
determined for all aquatic ecosystems. Thus, if the
resource quality objectives include the conservation of a
buffer strip of vegetation, then this will be achieved
through the legislation.
In terms of burning practices, there is a vast body of
research on the desired frequency and intensity of
burning that is required for maintaining the natural
4.5.4 Erosion, sedimentation and soils
Unsustainable land-use practices of the past have led to
erosion, sedimentation and the alteration of soil
characteristics throughout the country. Erosion-control
programmes have rarely been successful in South
Africa, as these were imposed on farmers who were
expected to put in labour without obvious benefits
(Booth 1994). In addition, many of these programmes
aimed at treating the symptoms of erosion, rather than
the causes of soil degradation. The development of
59
detailed soil erodibility and sediment yield maps is
expected to increase the efficiency of soil conservation
efforts. This would be greatly assisted by protection of
wetlands, sponges and riparian vegetation (Heeg et al.
1986).
The concept of integrated catchment management
incorporates land-use into the management of aquatic
ecosystems (DWAF 1996b). This approach should lead
to more sustainable land-use in the future, with the
quality of the aquatic resource being a priority.
maintenance costs of irrigation schemes, and paid
minimal amounts for water used. The substantial
proportion of the country’s water used for irrigation has
forced re-consideration of this policy, and in the Water
Act of 1998, no such subsidisation shall occur.
Irrigation farmers will pay capital costs, and the full
operation and maintenance costs, and will pay a realistic
price for water. Previously disadvantaged farmers will
be exempt from payment of capital costs, and water
tariffs will be introduced over a ten-year period
(Commission on Sustainable Development, 1998).
Apart from the occasional introduction of water
restrictions, mainly in the Eastern Cape, Gauteng and
KwaZulu-Natal, water use has been largely unrestricted.
Water supply was determined by water demand.
Recently, however, water managers have heeded the
call of many who for many years have urged the
Department of Water Affairs and Forestry to be more
cautious with water supplies, and aim to reduce the
demand, rather than merely meeting it. The concept of
water demand management, which manages demand
rather than supply, is now an accepted policy, and a
requirement of the new Water Act.
4.5.5 River regulation
The Water Act of 1956 was based on Roman Dutch
Law, which was developed in countries that did not lack
water. Thus, very little protection was given to the
aquatic resource itself, and concepts such as private
water and riparian rights were embodied in the Act
(O’Keeffe et al. 1992). Private water referred to water
that originated, fell onto, drained or was led onto private
land, and which could not be used for common
irrigation schemes, and was thus available for sole use
by the owner of the land. Riparian owners, owning land
adjacent to surface water, were entitled to “reasonable
use of normal flow”, but also had ownership of the land
that extended to the edge of the waterbody, including
the beds of rivers. Landowners could utilise water
without any specific obligations to downstream users.
This meant that there was no protection of the riparian
zone or the banks of the waterbody, and no provision
for downstream users.
The new Water Act of 1998 has replaced riparian
rights with the principle of beneficial and efficient use
in the public interest (Government Gazette 1998). The
Act provides special protection to the aquatic ecosystem
by reserving the amount of water and the flow regime
necessary for the maintenance of ecosystem
functioning, within the ecosystem itself. This is known
as the Ecological Reserve. Thus, the flow requirements
of the ecosystem must be determined before any
resource development or allocation of water rights are
allowed.
4.5.8 Biotic change
The Water Act of 1956 stated that it was an offence to
pollute any waters in such a way that it would harm fish
or any other aquatic life. It was also an offence to
cultivate, possess, convey, buy, sell, donate or import
certain aquatic biota, or to introduce them into any
waters (O’Keeffe et al. 1992). Each provincial nature
conservation body has authority over the provision of
permits for the introduction and transport of live fish.
However, no specific legislation has, until the recent
Water Act of 1998, protected aquatic biota from
activities which degrade aquatic habitats, such as flow
regulation (O’Keeffe et al. 1992).
The new Act is fairly specific about the conservation
of surface water resources as ecosystems, which should
provide adequate protection for the biota.
4.5.6 Habitat loss
Apart from the designation of reserves, there has been
no protection of ecosystems that occur on private land
(O’Keeffe et al. 1992). The exception is the Mountain
Catchment Areas Act (1970) which provided for the
protection of specific mountain catchments around
South Africa. The new Environmental Conservation
Act (1997), however, requires an assessment of the
impacts of any development which will affect surface
water resources, according to recognised techniques of
integrated environmental management (IEM).
4.5.7 Population pressure and water use
In the past, the use of water by irrigation farmers was
subsidised to a large extent by the government. Farmers
were exempt from payment of operation and
60